The principle of electromagnetic position detection is well known for determining the position and orientation of a solid moving body with a reference mark. One of the applications of this principle is the determination of the direction of sight of a helmet-type viewfinder which an infantry soldier, an operator, a tank driver or aircraft pilot puts on his head to control a weapon, missile, or navigation camera, for example.
This principle of detection uses a magnetic field transmitter or radiator, which is connected to the reference mark or reference frame in which the measurements are carried out, a magnetic field receiver or sensor, which is fixed to the moving body whose position and orientation are to be determined, and electronic processing circuits including analog amplifiers, a computer and processing algorithms.
The radiator must satisfy the conditions of dipole theory, in which the system of coordinates for the mathematical description of the radiation is a sphere centered on a dipole, and the Green function of free space depends only on the radial coordinate.
The magnetic field sensor must be as point-like as possible. The transmitter radiates a field, either sequentially or by multiplexing, along two or three orthogonal axes and the sensor detects sequentially the components of this field along three or two orthogonal axes, transmission and reception each generally being effected along three axes. The sensor thus provides, by way of the transmission axis, three measurements, that is to say a total of nine, which are organized into a 3.times.3 matrix, using the processing algorithms which provide the position and orientation of the sensor with respect to the radiator.
It should be noted at this point that the determination of the position and orientation of the sensor implies the determination of six variables--the three Cartesian coordinates, the relative bearing, the elevation angle and the roll--and that at least six measurements are thus necessary. If transmission is only effected along two axes, reception must consequently be effected along three, and vice versa.
FR-A-2 458 838/7914441 describes a system for implementing the principle of electromagnetic position detection. It could usefully be noted above all that the Applicants intend to consider this document as an integral part of the present application. Schematically, the radiator and the sensor each comprise a group of identical current-controlled identical coils which are disposed along the three orthogonal axes respectively. In order to best combine the conditions of the dipole both for the radiator and for the sensor, the dimensions of the coils should be as small as possible. It is also of interest, still with the same aim, to replace each coil by three half coils through which the same current flows. It could also be considered to have as the radiator and sensor a sphere made of magnetic material surrounded by three orthogonal coils.
In fact, in the previously implemented systems for electromagnetic position detection, the radiators and sensors have not fully met the conditions of the dipole theory and the object of the present invention is to come closer to them.